Depends on how hard you’re planning to push but unless you’re aiming 15+s then pick what fits your playstyle the best.
Warrios is currently FOTM but most likely will get nerfed in next patches. its what you could imagine a warrior being - at least from my understanding - really good versus physical damage, arguably highest damage. I’d rate it medium in terms of difficulty
DK is probably second best - mainly depends on selfheals and on spot heals (so monk healer fits him well) but in necrotic weeks you’ll feel it being an issue. damage seems to be alright, you get grips/mass grips which is helpful and if you manage your defensives well you shouldnt suffer in M+ or HC raids, more problematic in Mythic raids since after u’re out of defensives you depend on externals a lot, same goes for M+ if your defensives are down. its pretty hard to play tho, regardless of what people say. your defensive management and timing your spells well etc. is a key to being good. could add some more but you get the point. i’d rate DK hard
Monk is typical, hit and run playstyle, not sure about details but its more flexible and requires more kiting at least from what I saw, since i never really played it. I’d rate it probably hard, since it requires a lot of movement and good management of adds.
DH used to be highest/second highest damage but havent seen one for a looong time, not sure how they work either but I know they’re pretty weak right now. so I guess skip on this, cant really rate them but in the past they were pretty easy to play.
Druid tank - is probably the tankies versus physical damage but really weak versus magic damage, so say, on tyranical waycrest manor you can easly tank goliath boss without reseting its stacks but in motherlode, you get melted on bosses damage wise. I’d rate it easy regardless simply because of how beefy he is.
and pally tank is probably most reliable one, pretty easy to play, A LOT of survivabilty and goes well with priests, you got good amount of CC, and you’re reaaaally beefy. you just stand there and tank pretty much, rarely saw pallys even bothering to kite so ill rate it easy as well.
personally, I play DK, and once you understand spec well, its a lot of fun so I’d recommend it for sure. its pretty reliable as well, unless blizzard keeps nerfing them. you got good tools in kit, combat ress, grips, and decent damage, and with correct play you can pull half the dungeon and stay alive for 15-20 seconds.